A buyer ordered an expensive book. I just gave them the tracking number yesterday and today they open a return request (they haven't even received the item yet). They open a return stating the following:

"Return Reason:

Received extra item I didn’t buy (no refund needed)

Buyer Comment:

The book that I ordered originally arrived."

How could they have received an extra item when they didn't even receive the item from me yet? The item was just shipped yesterday? Are they trying to return something else and get a refund? This doesn't make much sense. How do I stop them from getting a refund?

Please help as I do not want them to get this book for free and return something else under the guise of receiving an extra item and getting a refund from that.